(1) Field of the Invention
Tubular writing instruments, particularly the felt tip type, wherein an annular expansion chamber is defined intermediate the felt tip and the ink reservoir, so as to admit expanding ink or an air bubble.
The present invention relates to a felt tip pen, of the type having a cylindrical body supporting a writing tip in its anterior end. The cylindrical body includes an interior bore whose posterior end is connected with an ink or writing fluid reservoir, communicating with the posterior end of the writing tip, since the posterior end of the writing tip extends directly into the reservoir. The cylindrical body includes in its anterior end both an inner pressure equalization chamber and an outer pressure equalization chamber. These chambers are superposed with respect to each other and are interconnecting at the posterior ends. The outer chamber includes an axial grove communicant with ambient air.
2. Description of the Prior Art
A known writing device of this type (West German - GMS No. 19 47 421), has the general form of a tubular felt tip writing pen, i.e., a writing tube supports a felt insert, as a writing tip. A type of fall weight is provided within the interior bore, so as to be axially reciprocable within limits, while rotating about its longitudinal axis. This creates an annular space inside the interior bore between the exterior wall and the fall weight, and at the posterior end of the interior bore and bore cross-section decreases and, correspondingly, also, the cross-section of the fall weight decreases. Due to the capillary characteristics of the annular space formed in this manner, there is presented a considerable resistance to the entrance of an air bubble from the inner pressure equalization chamber. In addition, the air bubble entering into the interior bore from the inner equalization chamber can also become lodged within the restricted annular space, particularly in the area with the reduced cross-section. Thus, on the one hand the equalization of pressure is made considerably more difficult and, on the other hand, the flow of ink or writing fluid from the ink reservoir to the writing tip may be blocked.